Summary

Reasons for performing study: Prolonged equine exercise can cause hypochloraemic alkalosis and hypokalaemia secondary to the loss of hypertonic sweat. Movement of ions in and out of erythrocytes during exercise may help regulate acid-base balance and changes in plasma ion concentrations. The extent to which this happens during prolonged equine exercise has not been reported.

Methods: Six horses were trotted at ∼30% V̇O2max on a treadmill for 105 min. Arterial (a) and mixed venous (v) blood samples were collected every 15 min, and pre- and post exercise. Blood gases and plasma (pl) concentrations of sodium, potassium, chloride and protein were measured and their iRBC concentrations calculated and compared (P<0.05).

Results: PaCO2 decreased in all horses. pl[Cl-]v decreased and [HCO3-]v increased. Due to the exhalation of CO2 and chloride shifting, [HCO3-]a<[HCO3-]v, pl[Cl-]a>pl[Cl-]v and iRBC[Cl-]a<iRBC[Cl-]v. pl[K+]a and pl[K+]v both initially increased then decreased and horses were hypokalaemic post exercise. Both iRBC[Cl-]a and iRBC[Cl-]v decreased over the course of exercise but there was no change in the arteriovenous difference between them. There was no arteriovenous difference in pl[K+]. iRBC[K +]a>iRBC[K+]v. Conversely, iRBC[Na+]a<iRBC[Na+]v. pl[Na+]a<pl[Na+]v and [TP]a<[TP]v.

Conclusions: Significant arteriovenous differences in iRBC and plasma concentrations of chloride, potassium and sodium reflect the role that movement of ions across erythrocyte cell membranes play in regulating acid-base balance and plasma concentrations of these ions. Exhalation of CO2 has a major influence on this ion flux.